Method of rectifying a neutral salt heat-treating bath



Feb. 20, 1951 s. M. DE POY' 2,542,637

METHOD OF RECTIFYING A NEUTRAL SALT HEAT-TREATING BATH Filed May 10,1946 user/2005s QQ 5A LT BATH LEVEL GPA PH/TE ROD INVENTOR Stewart M. DeFuf ATTORNEY Patented Feb. 20, 1951 METHOD OF RECTIFYING A NEUTRAL SALTHEAT-TREATING BATH Stewart'M. De Poy, Dayton, Ohio, assignor to GeneralMotors Corporation, Detroit, Mich., a

, corporation of Delaware Application May 10, 1946, Serial No. 668,821

This invention relates to a method for reducing oxides in neutral heattreating baths containing molten salts.

It is therefore an object of the invention to provide a method forreducing oxides in molten salt baths used in the heat treatment ofsteels and the like.

In carrying out the above object, it is a further object to position agraphite or carbon rod in the bath, which rod causes reduction of metaloxides in the bath and simultaneously acts as a scavenger in the bathfor picking up the metals per se whereby the bath is maintainedsubstantially free of oxides or other inclusions.

A still further object of the invention is to provide a method ofrectification of molten salt baths wherein the bath is maintained in astate of relatively high purity for increasing the life of theelectrodes and improving the action of the bath.

In carrying out the above object, it is still a further object toprovide a method whereby carbon pick-up in the bath is reduced so thathigh speed steels and the-like have substantially no carbon losses whentreated in the bath.

Further objects and advantages of the present invention will beapparent'from the following description, reference being had to theattached sheet of drawings which show a section through a typical heattreating bath.

Heat treatment of steels, such as, high speed steels in molten saltbaths has many advantages. First, more even temperaturecontrol, can beobtained and maintained than is usually practical when using atmospheretype furnaces. Further, the molten salt bath prevents any oxidation ofthe steel which is often experienced in atmosphere type furnaces, due toleaks, etc., occuring in the furnace. 1

One disadvantage of the usual molten salt heat treating bath is thecarbon pick-up of the bath which causes decarburization of the surfaceof the steel. Thus any steel which is susceptible to decarburization isdifficult to heat treat in a molten salt bath with satisfactory re sultsunless some expedient is used to prevent the decarburization. I

The use of silica in controlled quantities withk 4 Claims. (Cl. 148-15)in' the bath has been found to be a satisfactory method for reducing thedecarburization of objects being heat treated therein but this silicaaddition has a deleterious effect on the electrodes and also forms aslude which accumulates at the bottom of the pot andrequires frequentrenewal of the salt; The action of the silica on the electrodes is noteconomical, since electrodes are eaten away rapidly and necessitatefrequent replacement. Other methods for preventing decarburization areknown but in each and every one of these methods sludges are formedwhichnecessitate frequent renewal of the bath.

I have found that a satisfactory method may be practiced by the use of asolid graphite or carbon rod immersed in the bath, which rod reducesmetal oxides in the bath and acts as 'a scavenger for metals in solutionwhereby the rod may be periodically removed, cleaned and replaced. Theuse of the graphite rod has no deleterious efiect whatsoever on theelectrodes and no sludge is formed in the bath. Thus, the life of themolten salt bath is greatly increased as is the life of the electrodeswhereby a more economical heat treating method is obtained. Similarlythe action on the parts to be heat treated is negligible from point ofdecarburization whereby no soft outer skin is formed since the carbonpick-up of the bath is less than has been experienced by the use of anyother method.

Specifically referring to the drawing, a cross section of a refractorytype pot 20 is shown, which pot includes an internal reservoir 22 ofrectangular cross section. The salt bath level is indicated at 24 andthis salt is heated by means of electrodes 26 and 28 immersed in thebath.

The electrodes are connected to a suitable current source not shown. Thegraphite rod used as a metal pick-up and oxide reducer, is shown at 30.One means of positioning the rod in the pot is to form notches orindentations of the walls of the pot as at 32 and 34. The rod 39 ispreferably pointed at 36 and may be backed up into the indentation 232and then the point '36 thereof inserted in indentation 34 whereupon therod is maintained in place. Obviously, the rod may be immersed in thesalt bath by any satisfactory expedient whereby easy removal forcleaning purposes may be efiected, the embodiment shown being merely oneof the many expedients which may be used.

The salt bath may be any suitable type of neutral salt which normallycauses decarburization of the steel such as, barium chloride, barium choride and sodium chloride, barium chloride and potassium chloride, etc.The bath is us ally heated to temperatures of from 1750 to 2400 F. foraverage heat treatment on high sneed type steels. The present bath isparticularly desirable with molybdenum type steels since the steel maybe hardened without any substantial molybdenum distillation, or loss.

Three baths checked over long periods of time for life of electrodes anddecarburization of steels being treated and condition of baths gave thefollowing results:

TEST #1 Bath with no rectifier added [Temperature of bath, 2,225 F.Original carbon content, 34% (test piece). Average electrode life, 2,650hours] 2,225 F. Average electrode life, 720 hours] Soaking Carbon V Ageof Time of Content in Loss of Bath Test Outside Carbon Index 1 PieceLayer Hours Minutes Per cent p,

TEST #III Bath with" graphite rod rectifier [Original carbon content,.80%, (test piece). Operating temperature 2,225 F. Average electrodelife, 3,000 hours (electrode still useable).]

S lrin i C b oa g ar on Age of Time of Contentin Loss of Bath TestOutside Carbon Index Piece 1 Layer Hours Minutes Per cent 240 5 80 0. 000' 240 10 78 0. 02 2. 5 240 15 77 0. 03 3. 75 480 5 '79 0. OI l. 25' 672 5 79 0. 01 1. 25 960 5 78 0. 02 2. 5 1, 344 5 78 0.02 2; 5 l, 344 1576 0. 04 5. 0

From the results of these tests, it is apparent, that no rectifier asnoted in Test #1; the electrode stood up Well enough but thedecarburization index was high indicating marked carbon loss at theouter skin of the test piece. When the silicon dioxide rectifier wasused,. the decarburization index was lower and the loss of carbon lowerin the outside layer.

However the.

4. electrodes were eaten away in a comparatively short space of time andas previously stated, a sludge built up in the salt bath which requiredrenewal of the bath or frequent cleaning there of. When using a graphiterod, all of the aforementioned difliculties were eliminated in that thecarbon index was maintained at a relatively low figurethe carborrloss atno time was excessive and the electrodes even after three thousand hourswere still in useable condition.

From these tests, it is apparent that the use of; the; carbonArod-i.greatly improves the operatiomof the bath both from an operational andan economical standpoint.

With .respecttdthe area of graphite rod in a pot,.-thisisbest-derived atby checking the decarburization index, although I have found that inabatlr l6'-by 16 by 21 inches deep that a 2 inch diameter rod 23 incheslong is very satisfactory as is noted in test number 3.

Another set of tests was run to show thehardness' at thevaryirig depthsfrom the surface of the piece" and 'a't'th'e core thereof; The hardnessis also an index of thedecarburization.

These tests are" as follows Vickers Hardness Age Soaking V of P-imeohRectified Bath- Piece 1 By- 10007" ;.0012 .003 Hours Minutes. From From"From Gore l SurIa'ee Surface Surface 7 360- 5 479 556 776- 590 5 r 465573 775 662 5 40c 406 780 384 5' 630 717 787 576 5 e11 717 785 744 5:479: 650 888 1,416 51 440- 493- 905' 1,-416- 10 465- 523- 935 As..mentioned hereinbefore, as the. reduction of oxides progressesaametallpickmp isexperienced on .the. graphite-rod and aboutevery threeto four hours .the rod-should beecleaned that is the metal scraped. off.for maintaining the efiiciency of the. scavenging and .deoxidizingeffects.

While the embodiments of thepresent invention as herein disclosedconstitute preferred forms,. it is. to. be understood that other formsmight be adopted, all comin within the. scope of 'the claimswhichfollow.

What is claimed is as follows:

1. Thestepsin themethodof-rectifying aheat treating. bathwhichwilllnormally decarburize the steel being. heat treatedtherein,comprising; providing a molten salt bathwhich-is heated by metalelectrodes immersed therein, immersing a solid graphite scavengingrodsaid. salt bath for reducing metal oxidestherein. during the heattreating. p'ro'cessand. for: collecting metals thereon,andlperiod-ic'ally. removing said scavenging. rod from .theabathandcleaning. the metals therefrom andthenreplacingthe. cleaned rod in thebath.

2. The steps in the method of rectifying .a neutral salt heat treating.bath that will normally deca'rburizecarbone.bearing. steels. being heattreated thereim comprising;- providing amolten saltbath thatis. heatedby.metal .electr.odes immersed therein. immersing .a graphite.rodtherein. .reducing metalpxides inthebath duringrtheheat-treatingprocess throughcontactthereof...

with said graphite rod, simultaneously collecting the reduced metals onthe rod, and periodically removing the rod with the reduced metalsthereon and replacing it with a graphite rod free of metals wherebycontinued rectification of the bath will occur.

3. In the method of rectifying a neutral salt heat treating bath heatedby metal electrodes and simultaneously maintaining the oxide contentthereof at a point below that which will decarburize the surface of thesteel being heat treated therein, the steps comprising; reducing metaloxides in the molten salt bath during the heat treating process by meansof a solid graphite rod immersed in said bath, collecting reduced metalson the graphite rod for scavenging the bath, periodically removing thegraphite rod from the bath and cleaning said reduced metals from saidgraphite rod and then replacing the graphite rod in the bath.

4. In a method of rectifying a neutral salt heat treating bath heated bymetal electrodes and simultaneously maintaining the oxide content of thebath at a point below that which will decarburize the surface of thesteel being heat treated therein, the steps comprising; reducing metaloxides in the molten salt bath during the REFERENCES CITED The followingreferences are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,799,945 Beck Apr. '7, 19311,876,732 Neuhauss Sept. 13, 1932 2,349,767 Solakian et a1. May 23, 1941FOREIGN PATENTS Number r Country Date 14,479 Great Britain June 27, 1905OTHER REFERENCES Molten Salt Baths, page 50, published by E. I. du Pontde Nemours & Co., Inc., 1946.

1. THE STEPS IN THE METHOD OF RECTIFYING A HEAT TREATING BATH WHICH WILLNORMALLY DECARBURIZE THE STEEL BEING HEAT TREATED THEREIN, COMPRISINGPROVIDING A MOLTEN SALT BATH WHICH IS HEATED BY METAL ELECTRODESIMMERSED THEREIN, IMMERSING A SOLID GRAPHITE SCAVENGING ROD IN AID SALTBATH FOR REDUCING METAL OXIDES THEREIN DURING THE HEAT TREATING PROCESSAND FOR COLLECTING METALS THEREON, AND PERIODICALLY REMOVING SAIDSCAVENGING ROD FROM THE BATH AND CLEANING THE METALS THEREFROM AND THENREPLACING THE CLEANED ROD IN THE BATH.